1. Field of the Invention
The present invention relates to a filter catalyst for purifying exhaust gases, such as those emitted from diesel engines and including particulates.
2. Description of the Related Art
Regarding gasoline engines, harmful components in the exhaust gases have been reduced securely by the strict regulations on the exhaust gases and the technological developments capable of coping with the strict regulations. However, regarding diesel engines, the regulations and the technological developments have been advanced less compared to those of gasoline engines because of the unique circumstances that the harmful components are emitted as particulates (i.e., particulate materials, such as carbonaceous fine particles, sulfuric fine particles like sulfates, and high-molecular-weight hydrocarbon fine particles, hereinafter collectively referred to as “PMs”).
As exhaust gas-purifying apparatuses having been developed so far for diesel engines, the following have been known. For example, the exhaust gas-purifying apparatuses can be roughly divided into trapping (or wall-flow) exhaust gas-purifying apparatuses and open (or straight-flow) exhaust gas-purifying apparatuses. Among these, clogged honeycomb structures made from ceramic (i.e., diesel PMs filters, hereinafter referred to as “DPFs”) have been known as one of the trapping exhaust gas-purifying apparatuses. In the DPFs, the honeycomb structures are clogged at the opposite openings of cells in a checkered manner alternately, for instance. The DPFs comprise inlet cells clogged on the downstream side of the flow of exhaust gases, outlet cells neighboring the inlet cells and clogged on the upstream side of the flow of the exhaust gases, and cellular walls demarcating the inlet cells and the outlet cells. The DPFs inhibit the emission of PMs by filtering the exhaust gases with the pores of the cellular walls to collect PMs.
In the DPFs, however, the pressure loss increases as PMs deposit thereon. Accordingly, it is needed to regularly remove deposited PMs to recover the DPFs by certain means. Hence, when the pressure loss increases, deposited PMs have been burned with burners or electric heaters conventionally, thereby recovering the DPFs. However, in this case, the greater the deposition of PMs is, the higher the temperature increases in burning deposited PMs. Consequently, there might arise cases that the DPFs are damaged by thermal stress resulting from such burning.
Hence, as set forth in Japanese Examined Patent Publication (KOKOKU) No. 7-106,290, Japanese Unexamined Patent Publication (KOKAI) No. 9-94,434 and Japanese Unexamined Patent Publication (KOKAI) No. 2001-79,391, continuously regenerative DPFs have been developed recently. For example, in the continuously regenerative DPF disclosed in Japanese Examined Patent Publication (KOKOKU) No. 7-106,290, a coating layer comprising alumina is formed on the surface of the cellular walls of the DPF, and a catalytic ingredient such as platinum (Pt) is loaded on the coating layer. In accordance with the continuously regenerative DPFs, since the collected PMs are oxidized and burned by the catalytic reaction of the catalytic ingredient, it is possible to regenerate the DPFs by burning PMs simultaneously with or successively after collecting PMs. Moreover, since the catalytic reaction occurs at relatively low temperatures, and since PMs can be burned when they are collected less, the continuously regenerative DPFs produce an advantage that the thermal stress affecting the DPFs is so less that the DPFs are inhibited from being damaged.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 9-94,434 discloses a continuously regenerative DPF in which a coating layer with a catalytic ingredient loaded is formed not only on the cellular walls but also in the pores of the cellular walls. When the catalytic ingredient is further loaded in the pores, the contact probability of PMs to the catalytic ingredient increases, and it is possible to oxidize and burn PMs collected in the pores. In addition, the publication describes an NOx absorbent to be further loaded on the coating layer. With such an arrangement, NOx absorbed onto the NOx absorbent are released as NO2 at high temperatures, and accordingly it is possible to further facilitate the oxidation of PMs by the resulting NO2.
However, it has been found out that a part of PMs in exhaust gases agglomerate while flowing from an exhaust manifold to the continuously regenerative DPF. Then, PMs have grown granularly to a particle diameter of from 1 to 10 μm or even more than 10 μm. The thus grown PMs are less likely to come into the pores of the cellular walls so that they have deposited on the surface of the cellular walls. Accordingly, it comes to be more difficult for PMs to come into the pores. As a result, the deposition of PMs on the cellular walls has enlarged sharply to increase the pressure loss. Moreover, the contact probability of PMs to the catalytic ingredient loaded inside the cellular walls has dropped sharply to remarkably lower the oxidizing rate of PMs.